This application claims priority to International Application No. PCT/DE99/03833 which was published in the German language on Jun. 15, 2000.
The invention relates to a method for detecting and correcting non-linearities in radio-frequency, voltage controlled oscillators, and in particular, radio-frequency, coltage controlled osillators in the form of microwave oscillators for radar applications in motor vehicles.
Radar technology is particularly suitable for course use in automobiles or in industry for the purpose of contactless determination of distance, speed, nature, presence or the like of objects. In this case, functional scope, measurement accuracy and costs for radar sensors are fundamentally dependent on the modulation method used and on the associated radar signal processing.
In the context of the FMCW radar principle (FMCW=frequency modulated continuous wave) frequently used for distance measurement, the quality of the measured value depends, among other things, on the frequency accuracy and stability of the microwave oscillator. In practice, these variables are affected in particular by temperature drift, noise and non-linearities of the oscillator, and therefore generally need to be monitored.
Contactless distance and speed measurement using radar has been known for a long time and originates from military technology. In the case of the aforementioned FMCW method, on the basis of which the method described in EP 0 727 051 B1 for operating a radar instrument works, a frequency modulated radar signal is transmitted which is received with a shifting phase and frequency. The measured phase and frequency difference, which is typically in the kHz range, is proportional to the object distance, assuming that frequency modulation takes place linearly over time. In practice, this prerequisite is not typically satisfied.
The non-linearities during frequency modulation are primarily caused by the voltage controlled oscillators, since the components thereof have a non-linear voltage/frequency characteristic curve. In addition, these oscillators have more or less pronounced phase noise which has a much higher frequency (compared to the voltage dependent non-linearity) and is generally negligible at short distancesxe2x80x94the xe2x80x9ccorrelation lengthxe2x80x9d of the radar.
By contrast, it is necessary to compensate for the voltage dependent non-linearities in order to be able to carry out troublefree object detection using the radar device. Since the non-linearities change as a result of, for example, effects of temperature or aging, correction needs to be constantly adjusted in order to keep linearity differences in the tolerance range of a maximum of 1% of the measured value.
Appropriate correction methods are known from the prior art:
An oscillator can be driven using a once pre-determined, pre-distorted control voltage. This method is suitable only to a limited extent, however, since manufacturing scatter of the oscillator and also subsequent frequency fluctuations and temperature drifts cannot be compensated for, owing to the design.
The technical paper by P. Lowbridge et al. xe2x80x9cA Low Cost mm-Wave Cruise Control System for Automotive Applicationsxe2x80x9din Microwave Journal, October 1993, discloses the use of a control loop comprising a PLL (=Phase locked Loop) or AFC (=Automatic Frequency Control) circuit. In both methods, a frequency dependent reference voltage is produced which, in combination with a linear ramp, adapts the driving voltage of the oscillator such that the frequency modulation has a linear course over time. Such control electronics have the drawback of being too expensive and inflexible, however.
The technical publication by Nalezinski et al. xe2x80x9cNovel Heterodyne 24 GHz FMCW Radar with High-Precision 2.4 GHz SAW Reference Pathxe2x80x9d in MIOP""97 Conference Proceedings discloses the use of a reference path within the radar arrangement which, on account of its defined and precisely known delay time, corresponds to the disturbance-variable-free reference signal of a virtual reflection point whose distance is determined by a nominal delay time. Analysis and evaluation of this signal allows correction of real signals using similar delay times. However, this method is disadvantageous because the additional reference path and its associated evaluation unit are operationally complex and therefore cost intensive. The latter drawback is not acceptable, particularly with regard to the use of such radar sensors as a mass produced component in motor vehicles.
In one embodiment of the invention, there is a method for detecting and correcting non-linearities in radio-frequency, voltage controlled oscillators. The method includes, for example, tapping a frequency signal off from one of the oscillators, supplying the frequency signal to the first input of a logic EXOR gate, supplying simultaneously the frequency signal to a delay element, producing a delayed frequency signal, and supplying the delayed frequency signal to the second input of the logic EXOR gate, converting the digital pulses produced at an output of the EXOR gate into a DC voltage value proportional to the frequency of the oscillator frequency signal using a low pass filter; and generating a correction value for frequency control of the oscillator on the basis of the DC voltage value in order to set the oscillator frequency to a nominal value.
In one aspect of the invention, the frequency signal tapped off is divided down by a prescaler value using a frequency divider.
In another aspect of the invention, the frequency signal tapped off is divided down from a frequency in the GHz range to a frequency in the MHz range using the frequency divider.
In still another aspect of the invention, an edge profile of the frequency signal is steepened using a comparator.
In yet another aspect of the invention, the frequency signal is supplied to the EXOR gate via a multiplexer chip.
In another aspect of the invention, feeding at least one reference frequency signal derived from the radar sensor into the circuit in order to convert the frequency signal into the DC voltage value.
In still another aspect of the invention, feeding and processing two different reference frequency signals having frequencies in the range of the frequency signal into the module comprising EXOR gate and delay element via the multiplexer chip.
In another aspect of the invention, the correction value for the frequency control of the oscillator is produced using a microcontroller.
In yet another aspect of the invention, the reference frequency signals are derived from reference signals available in the microcontroller.
In another aspect of the invention, feeding and processing two different reference frequency signals having frequencies in the range of the frequency signal into the module comprising EXOR gate and delay element via the multiplexer chip.
In still another aspect of the invention, the reference frequency signals are derived from reference signals available in the microcontroller.